It has long been recognized that cardiac defibrillation --the termination of an episode of fibrillation--can be accomplished through application of an electrical shock to the cardiac muscle. See, for example, Swartz et al., "Influence of T-Wave Shock Energy on Ventricular Fibrillation Vulnerability in Humans," Journal of American College of Cardiology, 1995 Conference Abstracts, February 1995; see also, Karolyi et al., "Timing of the T-Wave Shock for Inducing Ventricular Fibrillation in Patients With Implantable Cardioverter Defibrillators," PACE NASPE Abstracts, Vol. 18, April 1995 (Part II), p. 802.
Numerous types of defibrillating devices, both external and implantable, are available for the purpose of cardiac defibrillation through electrical stimulation. One example is the Medtronic Model 7219 Cardioverter-Defibrillator, commercially available from the Assignee of the present invention.
One method of testing a defibrillator's operability to ensure that it is capable of reliably defibrillating the heart involves first inducing an episode of fibrillation in the patient's heart, and then activating the defibrillator to ascertain whether it is capable of terminating the induced fibrillation. A 60-cycle type fibrillator is one device used for inducing fibrillation, although such fibrillators are known to fail to induce fibrillation. Moreover, implantable defibrillator systems are becoming increasingly common, and it is not technically feasible or practical to incorporate a 60-cycle fibrillator into an implantable device.
To address these issues, it has been proposed in the prior art that fibrillation can be induced in either chamber of the heart (atrial or ventricular) by delivering a stimulus during that chamber's repolarization phase, i.e., within a so-called "vulnerability window" following the chamber's depolarization period when the heart has begun to repolarize but has not completely repolarized. This is described, for example, in U.S. Pat. No. 5,129,392 to Bardy et al., entitled "Apparatus for Automatically Inducing Fibrillation," which patent is assigned to assignee of the present invention and hereby incorporated by reference herein in its entirety.
According to the Bardy et al. '392 patent, the pulse intended to induce fibrillation is delivered in a timed relationship with an immediately preceding pacing pulse. An overdrive pacing and capture detection protocol is carried out (or some other method is used) to determine the patient's Q-T interval, enabling a subsequent fibrillation-inducing shock to be delivered at a time known to fall near the end of this interval but prior to the conclusion of the repolarization phase. The Bardy et al. '392 method and apparatus are believed to allow for extremely accurate placement of the fibrillation-inducing shock relative to the refractory period of the patient's heart.
The Medtronic Jewel.TM. cardioverter/defibrillator is an example of a commercially-available device which is capable of delivering a stimulus during the repolarization phase to induce fibrillation. The Jewel's so-called T-Shock.sup.198 feature requires the programming of three parameters: the rate at which overdrive pacing pulses are delivered (where overdrive pacing refers to pacing at a rate known to be above the patient's natural cardiac rate); the amplitude of fibrillation inducing shocks, and the shock coupling interval, which is the interval from delivery of the last overdrive pacing pulse to the delivery of the fibrillation-inducing shock. Clinical experience has shown that success in inducing fibrillation with the first T-Shock.TM. pulse occurs roughly 70-to 90% of the time.
Some clinicians may regard programming three parameters to accomplish automatic fibrillation induction as inconvenient or undesirable. Although a set of nominal or default parameters can be specified for the device, such nominal parameters are less likely to be appropriate for some patients, such as those on anti-arrhythmic drugs which slow conduction.
Another way that the success rate for inducing fibrillation might be improved is by increasing the energy content of the fibrillation-inducing shock. For example, a nominal pulse of 0.6J may be increased to 2.0J. However, this may have certain disadvantages, including minor irritation of the myocardium or increased pain for the unsedated patient.